The present invention relates to the preparation of slurries of solid coal particles in hydrocarbon liquid and is particularly concerned with the treatment of high sulfur coal to obtain a coal/oil slurry of acceptably low sulfur content wherein the hydrocarbon carrier liquid is a coal-derived oil.
The so-called "energy shortage" in recent years has given rise to extensive effort to find substitutes for previously utilized liquid and gaseous fuels derived from petroleum. Among the several avenues pursued is the utilization of abundantly available coal, either as a solid fuel or after its conversion to liquid and/or gaseous fuel products. A large part of the available coal is of undesirably high sulfur content (as much as up to about 3% or more sulfur by weight) giving rise to problems of environmental pollution, so that the relatively high costs of removal of sulfur from the raw coal, from the fluid fuels that can be derived from the coal or from the stack gases obtained in the combustion of the sulfur-containing solid or fluid fuels, is a major item in the cost of the obtained energy.
In modern coal treating plants the as mined coal (ROM) is crushed and separated into several size ranges for cleaning, comprising at least a coarse coal fraction, a fraction of intermediate size range and a fine coal fraction. Each fraction is separately cleaned in a manner deemed best for that fraction. In the typical cleaning operation a refuse fraction is removed from the remaining coal by gravity separation; employing such facilities as heavy media cyclones, fine coal jigs, Deister tables, or water-only hydroclones.
In some instances gravity separation may also be employed to further fractionate the good coal fraction, from which the refuse was removed, for recovery of a "deep-cleaned" float fraction of lowest sulfur and ash content.
Among the known methods proposed or used for initial treatment of crushed as mined coal is that generally designated "Multi-stream Coal Cleaning System" (MCCS) or 37 Multi-stream Coal Cleaning Process" (MCCP). The features of this system and process are described in Coal Age, January 1976 at pages 86 to 88, as presenting an advanced technology for removing sulfur from certain types of coal abundant in the eastern Appalachian coal fields.
By the use of a high density aqueous medium, such as a suspension of magnetite, the crushed coal is initially separated into (1) a refuse fraction which sinks in the medium, and which fraction contains a major part of the sulfur and minerals; and (2) a washed coal fraction of reduced sulfur and ash content. In a second step, conducted largely similarly to that of the first step, the already partly cleaned coal is further separated in a dense liquid medium at lower gravity into a sink fraction comprising coal of medium average sulfur content and a "deep-cleaned" low sulfur coal fraction (less than 1% by weight sulfur). Cyclones are employed in each of the gravity separation steps wherein the liquid medium is circulated at high velocity such that the lighter product rises to the top and is drained off, while the heavier fraction which sinks to the bottom, is there removed from the vessel. The aqueous medium is drained from the solid product and the magnetite is recycled into the cyclone.
A demonstration plant employing MCCP has been constructed and is in successful operation at the Homer City electric generating station in Indiana, Pa. The facility takes coal of about 2.6% sulfur and cleans it to obtain a middlings fraction containing about 2.2% total sulfur and a deep cleaned fraction containing less than 1% sulfur (about 0.8%).
While the conversion of coal into gas was an earlier established commercial technology in the United States to supply gas then needed for cooking and illumination, these gas plants were largely abandoned after World War II as pipeline transported natural gas came to be widely distributed. On the other hand, while conversion of coal to liquid fuel was in large scale operation in Germany, such process has not been employed on a commercial scale in the United States. In recent years, however, considerable research and development, funded by the U.S. government and by private industry, has gone into the development of scaled up technology for conversion of coal into oil and gas at prices competitive with the crude petroleum sources. Among the various methods proposed for liquefaction of coal, the better known are: carbonization, hydrocarbonization, direct hydrogenation, solvent extraction, the Fischer-Tropsch catalytic synthesis process, and the treatment of coal with oil to effect liquefaction.
The above processes are summarized and the literature articles describing the same in detail are identified in the introductory portion of U.S. Pat. No. 4,159,897. In addition to the citations in the aforesaid patent, among the more important systems proposed for conversion of coal to liquid fuels is that known as "SRC" (Sovlent Refined Coal) and a latter development of that process designated "Recycle SRC." These processes are described in a paper presented by Schmid, B. K. and Jackson, D. M. at the Fourth Annual International Conference on Coal Gasification, Liquefaction and Conversion to Electricity, University of Pittsburgh, August 2-4, 1977, under the title "Recycle SRC Processing for Liquid and Solid Fuels." Patents describing the SRC process or related similar processes, include: U.S. Pat. Nos. 3,341,447; 3,884,796; and 4,111,786.
In the original version of the SRC process (now generally known as SRC I) a distillate liquid is used for dissolving the coal, with subsequent recovery of the distillate for recycle as the solvent for the process. The primary product from this process is a low-ash, low-sulfur solid, designated "solvent refined coal." In the Recycle SRC process, a portion of the product slurry rather than a distillate liquid is employed as the solvent for the coal. A variation of the Recycle SRC process, now known as "SRC II," is discussed at length in a paper presented by Schmid, B. K. and Jackson, D. M. at the Third Annual International Conference on Coal Gasification and Liquefaction, University of Pittsburgh, August 3-5, 1976. In the SRC II process mode the dissolved coal is converted entirely to liquid fuel and by-product gases. By the several variations of the SRC process, one can produce distillate liquid, solid low-sulfur, low-ash coal, or combinations of these major products.
In all variations of the SRC processes the same general chemical reactions occur. These reactions include: dissolution of the coal in the solvent, hydrogenation of the dissolved coal to remove sulfur, nitrogen and some oxygen, and hydrocracking of the dissolved coal to form liquid and gaseous products. The undissolved mineral residue and heavy bottoms remaining after separation of the obtained liquid products is gasified by reaction with steam and oxygen, the product gas being then subjected to shift conversion and purification, thereby providing hydrogen for use in the SRC process.
Coal was also used for many years as the raw material source for production of synthesis gas for the chemical and fertilizer industries. However, in the late 1940's and 1950's, such use of coal was displaced by the increasing availability of natural gas. Here again, because of the crisis in the oil industry, wide spread activity was initiated for the development of coal gasification technologies, to provide desired gas products useful as clean fuel as well as for chemical feedstock. These developing new technologies were aimed at designing processes superior to those earlier employed. Among the better known of these later developed processes is that known as "Koppers-Totzek" Coal Gasification process (KT gasification). Another current coal gasification process is that called the Texaco coal gasification process (TCGP) and the closely related Texaco synthesis gas generation process (TSGGP). These processes are described in T.V.A. Symposium on Ammonia from Coal, May 8-10, 1979, at pages 72 to 85. The Winkler coal gasification process is described at pages 51-62 and 86-96 of the cited Symposium papers.
In the KT coal gasification process coal feedstocks of high ash and/or sulfur content can be utilized, contrary to the earlier solid-moving bed and fluidized bed processes. The finely dispersed coal is mixed homogenously with oxygen and optionally also with steam and fed to the KT gasifier through burners, wherein the coal particles are completely gasified. Part of the ash formed in combustion is discharged from the bottom of the gasifier as liquid slag. The remainder of the ash leaves the top of the gasifier as a fine dust, which together with small amounts of unconverted carbon is dispersed in the crude gas. After giving up heat to a waste heat boiler through which it is passed, the crude gas is cooled and most of the solid particles washed therefrom by a water spray. The further processing of the crude gas depends upon the ultimate intended use. In any event, sulfur removal is usually practiced, using any of the methods known in the art for treatment of acid gas. Carbon dioxide removal is common practice, generally with previously subjecting the gas to a CO shift reaction, particularly if increased hydrogen production is desired.
Except for particulars of operation, the general sequence in the other cited processes is largely similar to the previously described KT gasification. According to the above cited Symposium articles the gasification process can be employed in conjunction with a coal liquefaction operation, wherein the required hydrogen for liquefaction is produced from by-product residues. The composition of the gas obtained from the gasification of vacuum tower bottoms of SRC II operation, is given in Table 2 at page 84 of the Symposium paper.
Encouraging results have been reported in the partial substitution of coal for oil in boilers, wherein the coal in finely divided condition is suspended in the liquid fuel fed to a conventional oil-fired industrial burner. The initial coal-in-oil tests added pulverized coal to petroleum derived fuel oil. Later developments proposed as the liquid suspending agent for the pulverized coal, the liquid product obtained by liquefaction of part of the coal. Such operation is described, for example in U.S. Pat. Nos. 4,039,425 and 4,159,897.